Formation of chromium-containing coatings on both sides of steel strip with one coated side having a bright finish

ABSTRACT

IN A METHOD OF FORMING A CHROMIUM-CONTAINING COATING ON STEEL STRIP, A CHROMIUM-CONTAINING POWDER IS APPLIED UNIFORMLY TO ONE SIDE OF THE STRIP AND COMPACTED THEREON. THE THUS-PREPARED STRIP IS COILED AND DIFFUSION TREATED IN A PROTECTIVE ATMOSPHERE, PREFERABLY OF HYDROGEN, AND INCLUDING A HALOGEN-CONTAINING GAS AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO FORM A STAINLESS STEEL COATING ON BOTH SIDES OF THE STRIP

Nov. 30, 1971 J. v. D. FORSTMANN ETAL 3,623,901

FORMATION OF CHHOHIUM-GONTAINING COATINGS ON BOTH SIDES OF STEEL STRIPWITH ONE COATED SIDE HAVING A BRIGHT FINISH Filed Nov. 18, .1968

INVENTORS Julius v. D. Forslmann Richard M. Will/son United StatesFatent O US. Cl. 11722 Claims ABSTRACT OF THE DISCLOSURE In a method offorming a chromium-containing coating on steel strip, achromium-containing powder is applied uniformly to one side of the stripand compacted thereon. The thus-prepared strip is coiled and diffusiontreated in a protective atmosphere, preferably of hydrogen, andincluding a halogen-containing gas at a temperature and for a timesufiicient to form a stainless steel coating on both sides of the strip.

BACKGROUND OF THE INVENTION This invention relates to the formation of achromiumcontaining coating on sheet or continuous metal strip, and moreparticularly to a stainless steel coating on both surfaces of a steelstrip or sheet.

In the manufacture of chromized coatings by the method known as rollbonding, wherein a chromium-containing metal powder is applied to thesurface of a strip, compacted into the strip surface, and the articlethen subjected to a diffusion treating operation to produce a diffusedcoating of an iron-chromium alloy on the article, excellent stainlesssteel coatings can be produced on light gage strip. These coatings havecorrosion resistance comparable to that of stainless steel strip, andfor many applications the roll bonded coating is most acceptable.

A characteristic of the as-diffused roll bonded coating is thecomparative roughness of the surface of the coating. When a brightstainless surface is required for esthetic purposes, the roll bondedcoating must be applied in sufficient thickness to permit polishing,with consequent removal of a portion of the surface. To obtain asatisfactory product bearing a high polish may require repeating theprocess of applying powder, compacting and diffusing in order to providesufficient powder on the strip to obtain the thickness of coatingnecessary to withstand the final polishing step.

Accordingly, a principal object of this invention is to obtain achromized steel strip in which at least one side of the coated articlehas a bright finish.

Another object is to provide a chromized steel strip in which both sidesof the strip bear a stainless steel coating.

A further object is to provide a method of forming a bright finish on achromized surface produced from a roll bonded product without requiringa polishing step.

SUMMARY OF THE INVENTION We have found that by applying a uniform layerof chromium-containing metal alloying powder to one side of a strip andfollowing this with certain controlled treatment steps, a thin,adherent, ductile and corrosionresistant coating is produced on bothsurfaces of the strip.

In accordance with this invention, a steel sheet or strip is preferablycoated on one side with a thin film of liquid. The liquid should havesuch viscosity, volatility and tackiness characteristics as to render itsuitable as a temporary bonding agent for subsequently applied metalpowder. A chromium-containing metal powder is Patented Nov. 30, 1971 icenext applied uniformly over the filmed surface of the strip, and thestrip is then subjected to a rolling operation, or equivalent pressureapplication, to compact the powder. In this step, the powder is rolledinto a flat, compacted metallic layer in which adjacent grains of powderare bonded together. This metallic coating is in a semi-adherentcondition in relation to the strip, the underside of the metalliccoating having been mechanically bonded to the strip surface. Thecomposite article of strip bearing a compacted metal powder on onesurface is then given a diffusion treatment in a heat treating furnace,preferably in a coiled configuration in which a powder-coated surfaceand a non-powder coated surface are in close opposed relation. Thediffusion treatment is performed in a protective environment in whichthere is present sufiicient halogen-containing gas to promote transferof chromium from the powder-coated surface to the uncoated surface ofthe strip. Treatment is performed at a controlled temperature and for atime sufficient to produce an adherent, corrosion resistant stainlesssteel coating on both sides of the strip.

The effective carbon, hereinafter defined, in both the strip and theapplied powder, should be kept below certain predetermined limits inorder to develop coatings on the strip which are ductile and corrosionresistant.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a reproduction of aphotomicrograph of a transverse section of a steel strip coated on bothsides with an iron-chromium alloy.

DETAILED DESCRIPTION In one specific embodiment of the invention, afive-ton coil of 20 gage strip of titanium-stabilized steel, having atotal carbon content of 0.06%, was unwound from the coil, and a thinfilm of tridecyl alcohol was applied to one side of the strip by meansof rubber rolls. The strip was then passed horizontally through a fluidbed of finely divided Simplex ferrochrome powder mesh, US. StandardSieve Series), the powder being distributed uniformly on one side of thestrip. In this operation, the underside of the strip, that coated withalcohol, contacted the fluidized powder and was coated therewith. Thepowder had the following analysis:

Percent Chromium 69.8 Silicon 1.75 Carbon 0.035

Iron, balance.

The thus-treated coil was run through a two-roll temper mill to compactthe powder on the strip. The pressure of the rolls during compacting wassufficient to produce an elongation in the strip of about 2.0%. Afterleaving the rolls, the strip was wound on a take-up reel, unwound andrecoiled. During recoiling, a kinked wire spacer was inserted betweencoil convolutions to produce an opencoil effect. For most efficientdiffusion, it is desirable to space each lap of the coil to permit thediffusion atmosphere to circulate freely, and to prevent possiblewelding of adjacent laps.

The open coil was placed on edge in an annealing furnace of theLee-Wilson type, and, after properly sealing the furnace, an atmosphereof hydrogen and chlorine was introduced, as the coil temperature wasraised to a diffusion treatment temperature of 1650 F. The chlorinerepresented about 1.0% of the total diffusion treatment atmosphere, andwas fed to the furnace at a rate of 9 cu. ft. per hour. Hydrogen flowwas maintained at 900 cu. ft. per hour. About 15 hours were required tobring the coil to the diffusion temperature of 1650 F at which point theamount of chlorine being continuously introduced into the 3 furnace,along with hydrogen, was reduced to an amount equal to about 0.5% byvolume of the total treatment zone atmosphere. Once the 1650 F.temperature had been reached, the coil was allowed to soak at thistemperature in the hydrogen and chlorine atmosphere for about hours,during which time chromium in the ferrochrome powder-coated side of thestrip diffused into the strip, and iron diffused outwardly into thepowder coating. At the same time, chlorine in the treatment atmospherereacted with a portion of the chromium in the powder and transported itto the uncoated side of the strip, where the chromium was deposited andthen diffused into the strip.

Because the powder-coated side of the strip has a uniform distributionof powder at the outset, and thus permits a uniformly diffused coatingon that side, it likewise permits a uniform transport of chromium to thenon-powdercoated side by way of the even spacing of the laps of the coiland the even distribution of the treating atmosphere.

As there is a constant supply of hydrogen and chlorine to the furnace,there must also be a discharge of the gases. However, the mean retentiontime for any given amount of chlorine within the furnace is relativelyhigh, and th s chlorine used to transport chromium to thenon-powdercoated side, and iron from the strip to the powder-coatedside, may pass through a number of transport cycles in the confinedspaces between coil laps before this chlorine is exhausted from thesystem.

Introduction of chlorine was discontinued after 15 hours at diffusiontreatment temperature, to prevent further transport of chromium from thepowder-coated to the non-powder'coated surface. The soak period wascontinued for an additional 13 hours in a hydrogen gas atmosphere topermit the proper amount of diffusion of chromium and iron at bothsurfaces to form coatings of the desired thickness and composition. Inany event, chlorine should be exhausted from the atmosphere surroundingthe charge before the cooling cycle reaches that point at which chlorinecompounds condense to produce a deleterious effect on the surface of thealloy coating.

After cooling the strip in the furnace, during which a hydrogenatmosphere was maintained, the coil was removed, washed with dilutenitric acid and brushed to remove any loose powder or reaction productsretained on the coating surface. The strip was then given a temper rollon polished rolls equivalent to about 2.0% reduction.

The continuous, pore-free, stainless steel coating produced by themethod of this invention is adherent, ductile and corrosion resistant.The product has a matte finish coating surface on the previouslypowder-coated side and a bright finish on the opposite side of thestrip. The continuous, pore-free portion of the coating on each side ofthe strip ranged from 0.0011 inch to 0.0014 inch in thickness. Theproduct is illustrated in the drawing, which is a reproduction of aphotomicrograph of a transverse section of the coated strip, etched inpicral-nital, and taken at 100 magnifications. The bright surfacecoating is represented at A, while the matte surface coating isrepresented at B.

The non-powder coated side has a smoothness unattainable when both sidesof the strip are coated by applying powder to each before the diffusiontreatment, except by applying a double thickness of powder and thengiving the surface a high polish. Such means for obtaining a brightfinish would be very wasteful of powder. Our invention provides aninexpensive and efiicient method of producing a stainless coating onboth sides of the strip with one side having a bright finish. In manyapplications where a bright finish is required or desired, this type offinish may be necessary only on the side exposed to view.

It will be apparent that many alternative means or materials may be madeuse of in the various operating steps of the example. Alternatives incertain instances would cause little or no loss in efficiency.

It is not necessary that the strip be diffusion heat treated in coilform, although treatment of a coil is the most efiicient method from thestandpoint of handling and utilization of furnace space. The stripbearing the compacted powder on one side can be cut into convenientlengths of sheet, and the sheets can be placed vertically on edge in thefurnace with a powder-coated side and an uncoated side in facing, orclosed opposed, relation during the diffusion operation. Thismodification would require guides within the furnace to support thesheets in position.

Generally, if the steel strip stock is excessively soiled, it should becleaned with a cleaning medium such as a hydrocarbon solvent or analkali cleaner before applying the powder.

The powder-retaining material, which is applied to the strip in the formof a thin film, and which acts temporarily as a powder retaining medium,may be any liquid substance having the proper viscosity, volatility andtackiness characteristics previously referred to, and which, inaddition, leaves no carbon deposit on the steel surface, and meetssafety requirements. The metal powder can be applied to the steelbacking member without a liquid substance having the above-statedcharacteristics first being applied, but use of a liquid film ispreferred, for the film lends mechanical efficiency to the steps ofapplying and compacting the powder.

While not critical, it is desirable to control both the amount of liquidapplied and the grain size of the metal powder. The alcohol, or othersubstance, used for retaining the metal powder, should be applied in arather thin film of a thickness just sufficient to cause adequateadherence of the powder particles. An excess of the liquid may causeproblems of slippage and inefficient compacting during the rollingoperation.

In selecting a particular liquid as the powder-retaining medium, careshould be taken to select one which will not leave any substantialcarbon deposit in the compacted metal, which in turn could producebrittleness of the resultant coating. Kerosene is an alternative liquidwhich has been used successfully as the powder-retaining agent.Transformer oil and straw oil may be used also, although with lessefficiency than either tridecyl alcohol or kerosene.

Metal powder which passes a mesh screen has been found to be verysatisfactory, although larger particles may be used. The size of powderparticles desired will depend somewhat on the physical manner by whichthe powder is applied. It has been found desirable to have the powderparticles in a granular or angular shape, rather than flattened orspherical, to obtain the best control of powder coating weight andadherence of the compacted powder layer to the strip.

If the powder is applied to the strip in an amount of from 8 to 10 gramsper square foot of backing strip surface, quite satisfactory results areobtained, and this amount of powder is held on the strip readily by avery light film of tridecyl alcohol. Heavier or lighter applications ofpowder may be used, depending to some extent on the desired distributionof chromium and the thickness of the diffused coating. By using aheavier alcohol film, the amount of powder applied can be increased byapproximately twice the 10 grams per sq. ft. figure given above.

While application of powder to the strip is accomplished satisfactorilyby passing the strip through a fluid bed of powder, alternative methodsfor this application include use of a vibrator dispenser,electrophoretic deposition and electrostatic spray technique.

When the powder has been compacted onto the base steel, it is in theform of a porous shell which is held mechanically to the base. Porosityof the shell is advantageous, for, during the heating-up period prior todiffusing, the volatile, oily liquid, originally applied to hold thepowder to the strip, is vaporized and escapes through the pores of thecompacted layer.

The different types of powder contemplated for use in this invention arethose containing chromium, or iron and chromium. Metal powders answeringthis description are iron-chromium alloy, a mixture of iron and chromiumand commercial grade chromium powder. In the case of alloy powders, itwill be apparent that iron or chromium powder may be added if desired.Small amounts of metallic impurities, which do not affect the resultantcoating, can be tolerated in the powder.

We have found that excellent results are obtained with powder of thetype given in the process example, i.e. ferrochrome powder containingapproximately 70% chromium with the balance substantially iron. Thistype of powder produces, upon difiusion treatment, a stainless steeltype coating, which coating, when continuous and pore-free, will resista boiling 20 volume percent aqueous solution of nitric acid (based on100% HNO While a relatively high amount of chromium in the powder ispreferred for efficient formation of the coating, a stainless steelcoating can be obtained when the chromium in the powder representsconsiderably less than 70% of the total powder. Both the amount ofpowder applied and the amount of chromium required in the powder dependon the desired thickness and chromium composition of the coating.

The temperature during diffusion tretament should range preferablybetween approximately 1550 F. and 1900 F. for not less than about 12hours, although considerably longer times may be desirable, depending onthe amount of alloying required. Actually, there is no upper limit fordiffusion temperature other than that which may be dictated by practicalconsiderations. At temperatures above 1550 F., the minimum time requiredwill be lowered in an inverse manner.

The coating on each side of the strip, resulting after the diffusiontreatment, will generally have a thickness of from about 0.001 to 0.003inch. This coating will contain not less than about 12% chromiumthroughout, and will be characterized by a sharp interface between thealloy coating and the metal therebelow. Beneath the interface, thechromium content of the steel drops rapidly to zero.

Preferably, our coating has an average chromium content ranging fromabout 15% to 25%. Higher chromium contents may be used, but there wouldprobably be little or no added benefit from the standpoint of corrosionresistance.

In this invention, effective carbon previously referred to, is thatcarbon, either in the base steel or in the applied powder, which bydiffusion is free to combine with the chromium to form deleteriouschromium carbides in the coating. Stated differently, it is that carbonwhich has a greater affinity for chromium at the diffusion temperaturethan for other elements in the substrate or coating. If chromiumcarbides are present in the coating of the finished product insufficient amount, the coating is embrittled, and formability of thecoated product is limited. Furthermore, a chromium alloy coating,containing considerable chromium carbide, has lower corrosion resistancethan a coating substantially free of carbide.

Most metal powder will contain somecarbon, and this carbon must be heldto a value which will not produce the deleterious chromium carbides inthe ultimate alloy coating. The amount of carbon which may be introducedinto the compacted article by the powder should be not more than 0.25%by Weight of the powder used.

There is no limitation on the type of steel which may be used as basematerial in our invention, as long as the efiective carbon content ofthe base material is maintained at a figure no greater than 0.01% duringdiffusion treatment.

Maintaining the low value for effective carbon in the base steel duringdiffusion may be accomplished in various ways. The steel strip or sheet,for example a rimmed steel of 0.06% carbon, may be decarburized to below0.01% carbon before any of the processing steps of the invention areapplied.

Another procedure for obtaining the low effective carbon value in thebase steel during diffusion is to decarburize the powder coated strip inthe treatment furnace prior to the diffusion step. Successfuldecarburizing can be performed in this manner by introducing a moisthydrogen atmosphere (dew point, F., or 5.5% by volume) into the furnaceduring the heating-up period, then, when the temperature reaches about1250 F., holding at that temperature for about five hours. At the end ofthe five-hour period, the furnace is purged of the moist hydrogenatmosphere, and dry hydrogen is introduced. The required amount ofhalogen-containing gas must also be present in the diffusion treatmentzone before the diffusion temperature is reached, to produce thechromized coating effectively.

A third means, by which the effective carbon can be maintained at orbelow 0.01% during diffusion treatment, is that shown in the specificdetailed example of the process, wherein a titanium-stabilized steel isused as the substrate. Titanium is a carbide-former having considerableaffinity for carbon, and acts as a carbon-sequestering agent, and inthis manner carbon is tied up and is not free to migrate to the chromiumin the coating. Examples of other sequestering agents are zirconium andcolumbium. When carbide-formers, or sequestering agents, are used in thestrip base metal to tie up carbon in this invention, it is stillessential that any unbound, effective carbon in solution in the strip,that which is free to react with chromium in the compacted powder, beheld to a quantity not in excess of 0.01%.

When the carbon in the base steel is greater than 0.01%, and titanium isused to combine with the excess carbon, the amount of titanium necessarywill, of course, depend on the amount of carbon to be sequestered. As apractical matter, when using a titanium-stabilized steel, it isdesirable to maintain the titanium in an amount ranging from 0.2% to0.5%, preferably in the range of from 0.25% to 0.35%, and always in anamount at least four times the amount by weight of carbon it isnecessary to sequester.

In the specific process example of this invention, the base strip had ananalysis of 0.3% titanium and 0.06% carbon. This amount of titaniumcombines with substantially all of the carbon to form a stable titaniumcarbide. An advantage of using titanium-stabilized steel strip, overdecarburized strip, is in the fact that the stabilized strip has thestrength characteristics of a low-carbon steel.

Regardless of the source of carbon, the coating of the chromized productshould contain not more than 0.10% carbon. This refers especially to themain body of the coating. Carbides at the coating surface only may ormay not be detrimental.

The diffusion treatment must be performed in a protective atmosphere orenvironment including a halogen-containing gas and substantially free ofcarbon, oxygen or nitrogen. To this end, any one of the noble gases maybe used as a surrounding atmosphere along with the halogen-.

containing gas, although a more practical atmosphere is one composed ofsubstantially pure hydrogen and the halogen-containing gas. Hydrogen hasthe added advantage of being able to remove oxygen from oxides which mayhave formed during processing.

When hydrogen gas is used along with the halogencontaining gas, itshould preferably be pure hydrogen. However, even when allegedly purehydrogen is used, certain impurities may enter the furnace atmosphere inlarge scale operations, through leaks in the system, from the furnacewalls or other portions of the equipment or, possibly, in the hydrogenemployed. Chromium has a strong affinity for carbon, nitrogen andoxygen, any of which might find its way into the treating atmosphere.Oxygen will normally be the chief source of trouble.

At the diffusion temperature, and even considerably below suchtemperature, the highly reactive chromium powder reacts with any smallamount of oxygen present in the atmosphere, and the resultant chromiumoxide may completely surround the exposed portions of the powderparticles. The formation of the oxide shell on the particles hinders thenormal diffusion of the chromium particles into the steel base. For thisreason, when oxygen is present as impurity in the treating atmosphere,it may be necessary to provide a means whereby the chromium powder isfreed of its oxide and can then diffuse readily with the iron, both inthe steel strip base, and in the powder it self in the case where thereis iron in the powder.

Inclusion of a halogen-containing material in the furnace atmosphere hasbeen found to promote the rapid diffusion of chromium into the iron and,contrari-wise, the iron into the chromium, even when oxygen impuritieshave been introduced into the furnace inadvertently. Halogens, orhalogen compounds, act as scavengers, or energizers, in that they removethe oxide film from the chromium powder particles, ensuring metal tometal contact.

VVhiIe any halogen or halogen compound may be used as the energizerwhich is volatile at the diffusion temperature, or a few hundred degreesFahrenheit below the diffusion temperature, it is preferable to use asthe energizer, one which can be introduced as a gas at a relatively lowtemperature. When the energized is introduced in gaseous form, theamount and rate of introduction can be closely controlled. Hydrogenchloride, iodide, bromide or fluoride gas, among otherhalogen-containing vapors, may be used for this purpose. Suitablehalogencontaining materials, in the form of solid compounds, which maybe inserted into the furnace in solid formgand which volatilize at ornear diffusion temperatures, include ammonium chloride, chromicfluoride, and a monium bifluoride. Of the halogens themselves, chlorinegas has been found to be especially advantageous when injected in gasousform into the hydrogen atmosphere in the treatment furnace.

In this invention, the halogen-containing gas has an additional andmajor function in the treating atmosphere. The halogen component reactswith chromium on the powder-coated side of the strip, and transports thechromium as a gaseous chromium halide to the uncoated side of the strip,where chromium is deposited as it is displaced from the halide by iron.The newly formed iron halide returns to the powder-coated side of thestrip, again forming chromium halide, and this process of exchange isrepeatedly indefinitely during diffusion.

As shown in the specific example, the halogen-contain- 'ing gas shouldbe introduced into the furnace during the heating-up period to provide aclean powder surface and effective transport of chromium at the outsetof diffusion. Also, all halogen-containing gas should be purged from thefurnace before any solid halides are deposited on the strip surfaceduring cooling of the coil.

As an alternate procedure to introducing the halogencontaining materialinto the furnace as a gas, a solid halide may be introduced between thecoil convolutions before the furnace inner cover is installed. The solidmaterial will volatilize with the increasing temperature of the furnace,and will perform the same functions as the halogen-containing compoundwhen introduced into the furnace in gaseous form.

Because of the poisonous and corrosive nature of the halogens, properprecautions should be taken to prevent escape of these materials intothe ambient atmosphere, by using approved dispensing equipment and byinstalling proper venting facilitties on the treatment furnace.

Care should be exercised in the use of halogens to avoid formation of anexplosive mixture with hydrogen. For example, in Bureau of MinesBulletin No. 503, entitled Limits of Flammability of Gases and Vapors,it is shown that chlorine and hydrogen are known to produce an explosivemixture when the chlorine content of the mixture is above 11.0%. Whileit may thus be possible to use chlorine in an amount up to 9 or 10% ofthe treating atmopshere, for practical operations a halo- 8 gen contentbetween 0.10% and 1.0% has proved entirely satisfactory.

It has been found that in certain instances, an erosive condition may becreated in the coating in the presence of an excessive amount ofchlorine or other halogencontaining gas. The introduction of thehalogen-containing gas in large amounts apparently creates a physicaldisturbance on sections of the open wound coil, and consequentlyproduces an uneven distribution of the alloy coating layer. While thisunevenness of the coating has no effect on the surface appearance, noron the resistance of the coating to boiling nitric acid in a statictest, it may produce localized areas where the coating is quite thin.Thin spots in the coating would limit the amount of deformation orsurface finishing which could be performed on the coated article, as thethin areas would tend to split or rupture more readily than theremainder of the coating.

As used herein and in the appended claims, a halogencontaining gas isone which may include a gaseous halogen such as chlorine, bromine, etc.,a halogen acid gas such as, for example, hydrogen chloride, or anormally solid halide such as, for example, ammonium chloride, ammoniumbifiuonide or chromic fluoride.

In the appended claims, percentages relating to chlorine or otherhalogen component of the halogen-containing gas are expressed as volumepercent. All other claimed percentages refer to weight percent.

We claim:

1. A method of forming a coating on steel strip which comprises applyingto one side of the strip a uniform distribution of metal powder groupconsisting of chromium, ferrochrome and chromium-iron mixture,compacting the powder on the strip, treating the strip in coilconfiguration in a protective atmosphere including a halogen-containinggas wherein the halogen represents not less than 0.1% of the protectiveatmosphere volume in a diffusion treatment zone for a time and at atemperature sufficient to cause diffusion between a portion of thecompacted powder and the powder-coated side of the strip, and to causetransfer of a portion of the chromium in said powder to, and diffusionof the transferred chromium into the uncoated side of said strip, and tothereby form a stainless steel coating on both sides of said strip.

2. A method of forming a coating on steel sheets which comprisesapplying to one side of said sheets a coating of metal powder of thegroup consisting of chromium, ferrohrcme and chromium-iron mixture,compacting the powder on the sheets, disposing the sheets in a diffusiontreatment zone with a powder-coated side of one sheet facing an uncoatedside of an adjacent sheet, diffusion treating the sheets and compactedpowder in said treatment zone in a protective atmosphere including ahalogenconta ining gas wherein the halogen represents not less than 0.1%of the protective atmosphere volume for a time and at a temperaturesufficient to cause diffusion between a portion of the compacted powderand the powder-coated side of the sheets, and to cause transfer of aportion of the chromium in said powder to, and diffusion of thetransferred chromium into the uncoated side of the adjacent sheet, andto thereby form a stainless steel coating on both sides of said sheets.

3. A method according to claim 1 wherein the effective carbon content ofthe steel base is maintained at not more than 0.01% during diffusion.

4. A method according to claim 3 wherein the protective atmosphere ishydrogen and halogen-containing gas.

5. A method according to claim 4- wherein the metal powder contains notmore than 0.25% carbon.

6. A method according to claim 5 wherein the halogencontaining gas ischlorine.

7. A method according to claim 4 wherein the amount of chromium in thecoating is not less than 12%.

8. A method according to claim 6 wherein the amount of chromium in thestainless steel coating is not less than 12%.

9. A method according to claim 4 wherein the strip is decarburized inthe treatment zone during heating prior to difiusion treatment in theabsence of halogen-containing gas, and halogen-containing gas is addedto the protective atmosphere after decarburizing but prior to diffusion.

10. A method according to claim 9 wherein the strip is decarburized inthe treatment zone prior to dilfusion in an atmosphere comprisinghydrogen and water vapor until the strip contains not over 0.01% carbon.

11. A method according to claim 6 wherein the metal powder is chromium.

12. A method according to claim 6 wherein the strip is in theconfiguration of an open coil during treatment.

13. A method according to claim 6 wherein the powder is ferrochromepowder.

14. A method according to claim 8 wherein the difiusion-treated strip iscooled, at least in the early stages, in a hydrogen atmosphere.

15. A method according to claim v11 wherein chlorine is removed from thediffusion treatment zone no later than the point at which chlorinecompounds condense in the cooling step.

16. A method according to claim wherein the stainless steel coatingcontains not more than 0.10% carbon.

17. A method according to claim 8 wherein the stainless steel coatingcontains not more than 0.10% carbon.

18. A method according to claim 17 wherein the steel strip contains acarbide-forming metal in an amount sufficient to maintain the effectivecarbon in the strip at not more than 0.01%.

19. A method according to claim 18 wherein the carbide-forming metal istitanium.

20. A method according to claim 8 wherein the diffusion treatmenttemperature is not less than 1550 F. for not less than twelve hours.

References Cited UNITED STATES PATENTS 2,836,513 5/1958 Samuel (I)l17--1'07.2 P X 3,061,462 10/1962 Samuel (II) 117107.2 3,163,553 12/1964Commanday et al. 3,222,212 12/ 1965 Samuel et a1. 3,312,546 4/1967 Mayeret a1. 3,340,054 9/1967 Ward et al. 117107.2 UX

FOREIGN PATENTS 1,513,264 1/1968 France 117107.2

OTHER REFERENCES Applicants Non-Pat. Citations, Abstract of BelgianPatent 711,292.

ALFRED L. LEAVITI, Primary Examiner I. R. BATTEN, Jr., AssistantExaminer US. Cl. X.R.

